The present invention relates to a device for heating an amount of liquid in a container using steam, comprising a steam source, a conduit for the passage of steam, said conduit having an inlet end through which the steam that comes from the source enters, and an outlet end through which the steam exits and flows into the amount of liquid to be heated, said second end being able either to be located outside the container or within the amount of liquid in the container.
The steam source from which the liquid heating steam is derived, preferably consists of the boiler in a coffee machine for both professional and home use.
The same device which by the present invention is referring to may be supplied with a mixture of steam and pressurized air, thereby causing both heating and frothing of the liquid, particularly when the latter is milk.
Namely, in the prior art, the steam conduit outlet end has a bulb-shaped terminal that can be removed from such end, with one or more nozzles through which steam exits.
These types of devices are generally known and widely used with coffee machines, which have the inlet end of the conduit connected thereto by a sealed joint, for steam to pass through the conduit to heat the liquid in the container.
The joint allows the user to place the conduit outlet end both outside and inside the container, and immerse it in the amount of liquid to be heated.
Examples of these devices, which are known in the field of coffee machines as steam wands, are disclosed in WO 03/092458 and EP 1776905.
Since the steam that flows through the conduit of the device is at a high temperature, sometimes even slightly above 100° C., because it preferably is overheated steam, the outer surface of the conduit, which is usually made of stainless steel, copper, brass or bronze also reaches a rather high temperature.
Therefore, such high temperature affects handleability of the conduit in placing the steam outlet end into the container and the liquid to be heated, as it involves serious burn hazards for the operator.
Furthermore, due to the high temperature reached by the conduit, when the liquid to be heated is milk, deposits build up on the submerged conduit portion, due to milk calcification, and these deposits are not easily removed during the ordinary device cleaning procedure.
In an attempt to at least partially obviate the above drawbacks, from US 2010/0154648-A1 it is known to make a steam conduit from a polymeric material having a low heat transfer coefficient, and coaxially place it in a tubular metal element, separate therefrom by an air gap, the tubular metal element being the structural member for connection of the steam device to the coffee machine and for handing the conduit as it is introduced into the container in which the amount of liquid, particularly milk, to be heated, is contained.
It is also known in the art, as disclosed by EP 1.527.721-A1, to provide a heating device in which the steam conduit is made of a metal material, particularly steel, such that it may ensure the structural functions required for connection to the coffee machine and handling by the operator, and is enclosed in a shell made of a polymeric material having a low heat transfer coefficient, such as polybutylene terephthalate (PBT), polyamide (PA), polyethylene terephthalate (PET), and tightly fitted against the metal conduit.
Nevertheless, both the above mentioned prior art technologies were found to be unable to completely solve the problem of maintaining the temperature of the outer surface of the device, i.e. the surface designed to be contacted by the operator, low enough to prevent the build-up of deposits in the conduit section immersed in the liquid, particularly when the latter is milk, without affecting heating effectiveness.
The problem is solved by a heating device comprising a steam source, a conduit for the passage of steam, said conduit having an inlet end through which the steam that comes from the source enters, and an outlet end, through which the steam exits and flows into the amount of liquid to be heated, said second end being either located outside the container or within the amount of liquid in the container, said steam conduit being coaxially arranged in a tubular element which axially extends substantially from the inlet end to the outlet end of the steam conduit from which it is radially spaced, thereby forming a first axially-extending annular gap, said first gap being closed at both respective axial ends on said tubular element, said tubular element being coaxially arranged in a sleeve, thereby forming a second axially-extending gap, said second gap being closed at both their respective axial ends on said tubular element.